Weight stack apparatus



FIG. 1 is a perspective view from the top, front and right side of a weight stack apparatus showing my new design;

FIG. 2 is a top view of the weight stack apparatus, the bottom view being a mirror image;

FIG. 3 is a front view of the weight stack apparatus; and,

FIG. 4 is a left end view of the weight stack apparatus, the opposite end being a mirror image. 

The ornamental design for an weight stack apparatus, as shown and described. 